Mineral wool rafter fill with habitable rooms - any experience?

[Hilldes]

Did not quite bottom this one in another more general thread.

 

Does anyone have any specific experience please where all these are true:

 

• Roof space is habitable rooms with vaulted ceilings
• The rafters are full filled with mineral wool
• There is no insulation above the rafters
• There is no sarking (boarding) on top of the rafters

 

If yes, please share how you ventilated the roof and the specific build up above the rafters.

[ProDave]

Is this an existing building built like this or something you are planning to do?

[Mr Punter]

I think you would need to line the underside of the rafters with 30mm PIR with a VCL.

 

Ventilation could be above Tyvek type membrane, ridge vents, soffit vents and rafter trays, tile vents etc.

[Hilldes]

3 hours ago, ProDave said:

Is this an existing building built like this or something you are planning to do?

Its my new build

[Hilldes]

2 hours ago, Mr Punter said:

I think you would need to line the underside of the rafters with 30mm PIR with a VCL.

 

Ventilation could be above Tyvek type membrane, ridge vents, soffit vents and rafter trays, tile vents etc.

 Thanks, got the underside details sorted, will be 50mm PIR and taped of separate VCL.

 

The bit I'm struggling with is maintaining a gap between the mineral wool between rafters and the breather membrane. Easy with PIR between rafters, but with Mineral wool, how to stop it riding up to fill the cavity between the counter battens. 

[ADLIan]

Denser mineral wool rolls and slabs normally have a well defined thickness (not like the fluffy loft insulation). Try manufacturers websites for best products and best install method.

[Temp]

If the rafters are fully filled then above that you need either..

 

Membrain

Counter battens

Tile battens 

Tiles

 

Or

 

Counter battens

Membrane with 25mm drape

Tile battens

Tiles

 

The first option needs a membrane that the maker allows in contact with the insulation. Last I looked kingspan make one.

 

I'd also put PIR and a vapour barrier under the rafters.

Edited March 18, 2021 by Temp

[ProDave]

16 minutes ago, Hilldes said:

Its my new build

The I would seriously consider External insulation above the rafters, turning it into a warm roof, rather than the extra insulation under the rafters.  The External insulation will double as a sarking board.

 

Then you can full fill the gap between the rafters without need to consider any ventilation.

 

Over the external insulation you need a "non tenting" breathable membrane and the gap between that and the tiles needs ventilating with eaves vents and a vented ridge.

 

I found Frametherm 35 was a good option.  A lot more solid than the yellow fluffy loft insulation and a lot less nasty to handle.  It is stiff enough to cut slightly over width and it will push in between the rafters and stay there.

[Hilldes]

51 minutes ago, ADLIan said:

Denser mineral wool rolls and slabs normally have a well defined thickness (not like the fluffy loft insulation). Try manufacturers websites for best products and best install method.

 

Thanks, planning to go for slabs such as ROCKWOOL Flexi Slab in two layers between the 225mm rafters - 140mm + 90mm, so when held in by the PIR across the internal face of the rafters should not push upwards too much into the cavity formed by the counter battens.

[Hilldes]

43 minutes ago, ProDave said:

The I would seriously consider External insulation above the rafters, turning it into a warm roof, rather than the extra insulation under the rafters.  The External insulation will double as a sarking board.

 Yes with hindsight that might have been a better option. The timber frame is being erected at present and we have planning constraints on the height of the eaves and ridge.

[Hilldes]

53 minutes ago, Temp said:

If the rafters are fully filled then above that you need either..

 

Membrain

Counter battens

Tile battens 

Tiles

 

Or

 

Counter battens

Membrane with 25mm drape

Tile battens

Tiles

 

The first option needs a membrane that the maker allows in contact with the insulation. Last I looked kingspan make one.

 

Option 1 - will look for a membrane that can be in contact with insulation. With this option though, I'm guessing the membrane would not be brought up over the eaves ventilation? So any water ingress from the tiles will not run into the gutters?

 

Option 2 - looks promising if I use mineral wool slabs and they don't push up into the cavity created by the counter batten. And the membrane can still pass over the eaves ventilation (or at least on top of the eaves rafter roll that comes with the eaves vent kit and then the rafter roll overlaps the gutter).

[Russell griffiths]

Who is fitting the rockwool?

having just worked on a house with rockwool fitted between the rafters I would safely say it is the nastiest building product you will come across. 

 

I would look at alternatives. 

 

I brought a few different manufacturers insulation just to test fit and found Knauf omnifit to be the nicest to use. 

[Temp]

 

 

11 minutes ago, Hilldes said:

 

Option 1 - will look for a membrane that can be in contact with insulation. With this option though, I'm guessing the membrane would not be brought up over the eaves ventilation? So any water ingress from the tiles will not run into the gutters?

 

Option 2 - looks promising if I use mineral wool slabs and they don't push up into the cavity created by the counter batten. And the membrane can still pass over the eaves ventilation (or at least on top of the eaves rafter roll that comes with the eaves vent kit and then the rafter roll overlaps the gutter).

 

If you have fully filled rafters you need a vapour permeable membrane and there is no void under it that needs ventilating.  If the tiles are very flat, solar PV or slates then i think they recommend the space above the membrane is ventilated. With irregular or hand made tiles there are enough gaps for ventilation.

 

Both options are designed to allow water to run down to the gutter. They aim to prevent the insulation pressing the membrane against the underside of the tile battens. That can cause water to pool on the up hill side of the tile battens. Pollen and dust can also collect there making amud like mixture that stops water running down.

 

Your BCO may have a preference for one or the other. 

[Hilldes]

Thanks @Temp the tiles are concrete interlocking Marley Edgemere and Marley recommend ventilation. Will seek advice from the BCO.

 

30 minutes ago, Russell griffiths said:

Who is fitting the rockwool?

having just worked on a house with rockwool fitted between the rafters I would safely say it is the nastiest building product you will come across. 

 

I would look at alternatives. 

 

I brought a few different manufacturers insulation just to test fit and found Knauf omnifit to be the nicest to use. 

 

I'm fitting the mineral wool. I had specced Frametherm 32 slabs but just can't get it and high prices. May go for Frametherm 35 but I think I might also struggle to source in slabs and at a reasonable price. Will look at Kanuaf Omnifit.

[Russell griffiths]

2 hours ago, Hilldes said:

Thanks @Temp the tiles are concrete interlocking Marley Edgemere and Marley recommend ventilation. Will seek advice from the BCO.

 

 

I'm fitting the mineral wool. I had specced Frametherm 32 slabs but just can't get it and high prices. May go for Frametherm 35 but I think I might also struggle to source in slabs and at a reasonable price. Will look at Kanuaf Omnifit.

I fitted 700m of the knauf stuff, I will never use rockwool again.